Our objective is an understanding of the means by which the diverse cell types of the limb bud arise from an apparently uniform population of limb mesenchyme cells. The following experiments are planned. 1) Transplantation of dorsal and ventral ectoderm will be performed to further test the role of ectoderm in muscle differentiation. 2) Electron microscopy and autoradiography will be used to study the synthesis and deposition of mucopolysaccharides in the muscle- and cartilageforming regions of the limb bud. 3) The synthesis of actin and myosin will be studied by incorporation of radioactive amino acids into proteins and their separation by acrylamide gel electrophoresis.